"We had no idea there would be 2 billion downloads by October," Kleiner Perkins partner Matt Murphy, manager of the then $100 million fund, told Financial Times. "Most people within Apple, if you had told them it would be a fifth of that by now, they would have been pretty happy."

Apple's initial reluctance to allow third-party native app development may have been influenced by those low expectations. When the iPhone was first launched in June 2007, Steve Jobs tried to sell developers on just making Web apps. When Web-based apps couldn't match the performance or capabilities of native apps, developers asked for more. Apple announced that they were changing course in October, but didn't launch the App Store until July 2008—just as Apple released the second generation iPhone hardware.

The success of the App Store is attributable to a sort of positive feedback loop among developers and users, FT explained. Developers created the 100,000+ apps that are currently available, giving users a smartphone platform that is capable of almost limitless functions. The growing user base that was attracted to the selection of apps then provides developers a large potential market to address with new applications.

"From Apple’s perspective, the gold mine was that they found a source of differentiation, independent of the hardware and basics software design, that they could never have anticipated," Professor David Yoffie, of Harvard Business School, told FT.

Besides benefitting Apple, some developers have also had quite a bit of success developing iPhone apps. Tap Tap Revenge maker Tapulous was recently profiled by Reuters, noting that the company generates about $1 million in revenue per month. While most developers won't be able to sustain such numbers, it shows that a bit of dedication, clever design, and more than a smidgen of luck, developers have a shot at making serious money.